Design and Validation of a Prototype Collimation System with Reduced Applicator Weights for Elekta Electron Therapy Beams
The objective of this study was to design an electron collimation system for Elekta Infinity accelerators with significantly reduced applicator weights, while maintaining acceptable in-field beam flatness and out-of-field leakage dose. The design was restricted to 6-20 MeV beams for 6x6 to 25x25 cm2 applicators at 100 cm SSD with the MLCi2 treatment head. In Aim1, measured off-axis profiles showed that the current Elekta collimation system met dose flatness (4%) and IEC leakage dose specifications in the patient plane for custom 7, 13, and 20 MeV beams at MBPCC. Also, Monte Carlo (MC) calculations based on MBPCC beam models sufficiently agreed with measured doses to be useful for collimation design. In Aim 2, fast, analytical models for the electron and photon dose components were developed for calculating dose inside the field and its penumbra during trimmer weight optimization. In Aim 3, 1% electron ranges and out-of-field leakage doses were both computed using MC for copper, lead, and tungsten. At 20 MeV tungsten and lead had comparable ranges (g cm-2), both lower than copper. However, tungsten had lower out-of-field dose, making it the preferred trimmer material. In Aim 4, MC calculations evaluated the effect of diverging the trimmer inner edges on out-of-field dose. Results showed that edges diverging from the virtual source for the upper and middle trimmers gave the least out-of-field dose; the divergence of the lower trimmers edges made little difference. In Aim 5, off-axis positions of the inner and outer trimmer edges computed by a simple model resulted in initial 10x10 and 20x20 cm2 applicator designs having excessive trimmer weights of 7.06 and 10.87 kg, respectively. Trimmer modifications (beveling outer edges, optimizing z-positions, optimizing edge positions, reducing photon jaw positions, and adjusting trimmer thicknesses and bevel shapes) reduced trimmer weights to 3.73 and 5.09 kg, well below the objectives (5.00 and 7.10 kg, respectively). In Aim 6, dose measurements with fabricated 10x10 and 20x20 cm2 prototype applicators confirmed acceptable in-field flatness and out-of-field leakage dose at 7, 13, and 20 MeV, achieving the objective. Also, photon jaw inner edges should be within 1 cm of the optimized locations.
electron collimation, electron radiotherapy
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